Possible links between the alleged plot to blow up transatlantic jets and the 7 July London bombings are being probed, sources have told the BBC.
Armed police have been out in many UK airports
Security correspondent Gordon Corera said officials also fear unconnected groups could be inspired to carry out their own attack.
Meanwhile Home Secretary John Reid said recent counter terrorism arrests did not mean the threat had ended.
Police continue to hold 23 people over the alleged liquid bomb plot.
Our correspondent said concern over the possibility of an attack meant it could be several weeks before officials felt confident enough to lower the threat alert level from "critical".
Mr Reid warned against "complacency or self-congratulation" as the threat posed by extremists was "ongoing", as were efforts to combat it.
The Metropolitan Police have warrants to detain 22 of the suspects until next Wednesday.
The Home Secretary warned against 'complacency or self-congratulation'
A hearing regarding the detention of one individual was adjourned until Monday and they will remain in custody over the weekend, while another person has been released with no further action.
The suspects were held following raids in London, High Wycombe in Bucks, and Birmingham, early on Thursday morning.
Those held are suspected of involvement in a plot to blow up airliners travelling from the UK to the US, possibly using liquid explosives hidden in hand luggage.
Airport security has been stepped up in both countries, with stringent luggage searches and restrictions on the carrying of liquids on board.
This has prompted concerns that valuable items such as laptops and MP3 players, which cannot be carried in hand luggage, may not be covered by insurance companies.
A spokeswoman for the Association of British Insurers advises passengers to avoid taking such valuables if possible, but added that insurers are being sympathetic and will treat any claims for damages on a case by case basis.
Officials in Pakistan said two British men of Pakistani origin have been arrested in connection with the alleged plot.
Pakistan's Foreign Ministry has identified one of them as Rashid Rauf.
He thought to be related to Tayib Rauf, 22, who was arrested in Birmingham, and is among 19 of the suspects who have had their assets frozen by the Bank of England.